The present invention relates to Optical Dispersion Compensation.
One of the main sources of signal distortion in optical fibers is chromatic dispersion. Since dispersion has a linear process and affects the signal phase, it can be perfectly compensated without inducing any penalties. There are three main approaches to overcoming distortions due to fiber dispersion. First, reducing link dispersion, and second, compensating for dispersion effects at the coherent receiver or at the digital transmitter using digital signal processing.
The first approach can be implemented by designing fibers that have almost no dispersion, such as dispersion shifted fibers. However, it is well known if fiber dispersion is reduced, nonlinear impairments increase significantly. Since it is much more easy, and less expensive to compensate for fiber dispersion, than compensating nonlinear impairments, this method has been largely discarded by the community.
Another way to reduce link dispersion is concatenating different fibers in the link such that the dispersion of one fiber compensates totally or partially the dispersion of the other fiber. Such links are called compensated links. However, the fibers having negative dispersion has higher losses and higher nonlinearities. Moreover, since the dispersion of the link is periodically compensated, similar to the case of using low dispersion fibers, these links also have higher nonlinear penalties compared to using uncompensated links.
The second approach is to use uncompensated link with coherent transceivers, and compensate the accumulated dispersion at the transmitter side or at the receiver side using digital signal processing (DSP). The second approach has two problems. First, link dispersion can be very large, and current DSP technologies may not be adequate for compensating for the accumulated dispersion of transoceanic links. Second, compensation of dispersion by DSP requires very large amount of power consumption. Indeed, even in current receivers, majority of the power consumption is due to dispersion compensation.
The third approach attempts to compensate for the total dispersion of the entire link at the receiver side or at the transmitter side by optical methods. Basically, a passive optical component having the opposite of the accumulated dispersion of the entire link can be placed at the transmitter or at the receiver to fully compensate for the entire link dispersion. This is sometimes referred to as lumped dispersion compensation (400). In this case signal does not experience increased nonlinear penalty during transmission because fiber has large dispersion. However, so far there is no optical component that can compensate for the accumulated dispersion of an uncompensated link more than a few hundred kilometers meters long with acceptable insertion loss.
Since dispersion is a linear effect, in principle these components can be cascaded many times to achieve the amount of total dispersion that is required. However, because of the large insertion loss and also large PDL of these components, cascading them require using additional optical amplifiers which both increases system cost and reduces signal quality because of additional amplifier noise, and additional polarization-dependent loss.
FIG. 1 shows one approach where optical components use fiber Bragg grating (FBG) for dispersion compensation (100). Bragg gratings (BGs) can be written on many media, and when they are written on fibers they are called FBGs. FIG. 1 shows a simple schematic of how FBGs are typically used for dispersion compensation. The dispersed light enters from the left port of the circulator (101) and it is directed to the FBG (102). The short wavelengths have a higher group velocity, therefore they are ahead of the other wavelengths, and longer wavelengths are trailing. The BG is written with a chirp in the pitch, meaning that the pitch changes along the grating so that a phase matching condition is satisfied for longer wavelengths at the near side of the BG but not for the shorter wavelengths. Hence, the longer wavelengths are reflected from the nearside of the FBG. Similarly, the pitch is designed so that it satisfies the phase matching condition for shorter wavelengths at the far side of the BG. As a result, if the pitch and the chirp in the pitch is controlled well enough, the shorter wavelengths can be delayed with respect to the longer wavelengths by just the right amount so that the dispersion can be compensated.
There are two limitations to this method. First the largest dispersion amount that can be compensated depends on how long the FBG can be written, and the maximum length is currently limited to only a few meters due to the limitations of the FBG writing techniques. Second, FBG is reflective. Therefore it is necessary to use a circulator, which increases the insertion loss by at least 1.7 dB, because of the dual pass through the circulator. Indeed, the circulator is the major contributor to the insertion loss since the loss of a few meter long fiber can be ignored and writing FBG does not increase fiber loss appreciably. Assuming a maximum of a 1 m long FBG, the maximum amount of dispersion that can be compensated for is only 10000 ps, which is far less than 50000 to 150000 ps which is required for typical uncompensated transoceanic links. Cascading several FBGs can achieve the desired total amount of dispersion compensation at the cost of increased insertion loss.